Technology for positioning mobile radio terminals using the signals received from one or more transmitters has been widely used for many years. Such systems include terrestrial networks of transmitters (e.g. LORAN-C) and networks of satellites (e.g. the Global Positioning System, GPS) deployed specifically for the purpose of locating the receiver, as well as methods that use general-purpose radio networks such as cellular mobile telephone networks (e.g. U.S. Pat. No. 6,529,165) or TV and radio transmitter networks. (e.g. U.S. Pat. No. 5,045,861).
Within a cellular mobile telephone network, for example, the position of the mobile terminal may be based on the identity of the serving cell, augmented by information such as the time delay between the serving transmitter and mobile terminal, the strengths of signals received from the serving and neighbouring transmitters, or angles of incidence of received signals. An improved position may be obtained using the observed time difference of arrival (OTDA) of signals received at the mobile terminal from two or more transmission sources.
OTDA methods give good position accuracy using only the signals available within a cellular radio network. However, in unsynchronised networks such as GSM and W-CDMA networks for example, they require the transmission time offsets of the transmitters to be determined in order to solve the positioning equations. This can be done using location measuring units (LMUs) having additional receivers. LMUs are placed at known locations so that their OTDA measurements can be converted directly into a network timing model (e.g. WO-A-00-73813).
Alternatively, a technique (U.S. Pat. No. 6,529,165) may be used in which measurements of signals from a number of geographically disparate transmitters at known positions made, for example, by two geographically disparate mobile terminals at unknown positions, may be used to compute simultaneously both the positions of the mobile terminals and all the timing offsets between the measured transmitters, without the need for LMUs.
A problem associated with these network-based positioning systems is the need to know the locations of the transmitters themselves. For example, the computing node at which the calculation of the position of the mobile terminal is made must have access to a database of the latitude, longitude, and height above a defined datum of every base station in the network. However, such information can be commercially sensitive, and therefore difficult to obtain, and even when it is available the database can contain errors. If the position calculation is carried out in the mobile terminal itself, the database (which may be substantial in size) must be sent to the mobile terminal thereby consuming communications bandwidth. Another problem is that the mobile terminal must be registered on the network in order to receive the information, and cannot therefore operate in an entirely autonomous mode.
The present invention is aimed at overcoming these problems by providing a method by which the position of a mobile terminal (such as one operating in a mobile communications network) may be found autonomously. The position of the mobile terminal can be calculated within the mobile terminal itself, albeit with reduced accuracy, without the need to have access to a database of base-station locations.